A lot of powerful boxer do hill sprinting. It is brilliant for increasing speed and power. Carl Froch especially is a fan.

The following circuit should be performed once and increased over time. It wears the legs like hell but the gains are brilliant. Find a hill and sprint the following. After reaching each landmark, walk back to the start; this is an active rest:

10mx5
20mx4
30mx3
40mx2
50mx1

Use the above first and once you improve you can go back (e.g. 50mx1, 40mx4, 30mx3 etc).